This invention relates to an inorganic fiber-reinforced heat-resistant ceramic composite material comprising a matrix of ceramics and inorganic fibers composed mainly of Si, either Ti or Zr, N and O as a reinforcing material.
Heat-resistant ceramics are used under severe conditions, for example at superhigh temperatures or superhigh pressures or in a corrosive environment, but have the defect of being weak to mechanical shocks and decreasing in mechanical strength or corrosion resistance at high temperatures. Attempts have been made to remedy this defect by developing cermets which are composite materials of metals and ceramics, and composite materials of ceramics with continuous filaments of fused quartz, alumina or carbon or with short fibers or whiskers of silicon carbide have been developed.
The cermets, however, cannot have sufficient high temperature strength and their service life is short because the metal component is susceptible to oxidation at high temperatures and has a lower softening temperature than the ceramic component. Furthermore, the range of their application is very much restricted. The composite materials of ceramics with continuous fibers of fused quartz or alumina, on the other hand, have the defect that the cost of production of these fibers is very high. Moreover, these composite materials have only a limited application because the fuzed quartz has a low modulus of elasticity and alumina has poor thermal shock resistance. The composite materials of ceramics with short fibers or whiskers of carbides or nitrides, such as silicon carbide, have the highest durability in a high-temperature oxidizing environment among the composite materials mentioned. But these fibers or whiskers cannot be obtained in uniform diameter, and lack homogeniety. Hence, the composite materials containing these reinforcing materials are not uniform in properties such as strength, and have low reliability in use. Furthermore, these short fibers or whiskers cannot be produced on a mass-production basis and the cost of producing composite materials of this type becomes high.
Carbon fiber composite materials which can be produced on a mass-production basis and are relatively easy to use economically still have the defect that they cannot be used in a high-temperature oxidizing environment.
Japanese Laid-Open Patent Publication No. 81309/1977 discloses the production of a heat-resistant ceramic composite material composed of a carbide or nitride ceramic as a substrate and silicon carbide fibers obtained from an organosilicon polymer as a reinforcing material. Japanese Laid-Open Patent Publications Nos. 169152/1981 and 169186/1981 dicsclose the production of a ceramic composite material composed of a glass or an aluminosilicate as a substrate and the aforesaid silicon carbide fibers as a reinforcing material.
Silicon carbide fibers obtained from these organosilicon polymers are the highest in mechanical strength such as tensile strength and modulus when the temperature at which fibers spun from these organosilicon polymers and rendered infusible are fired in an inert gas or in vacuum is 1200.degree. C. However, when the firing temperature becomes 1300.degree. C. or higher, the above mechanical strengths abruptly decrease. This decrease is said to be due to the fact that at a temperature of 1300.degree. C. or higher, microcrystals of .beta.-SiC occurs throughout the fibers and degrade them.